A number of imaging technologies are used in the high-quality color printing market. Among the leading imaging technologies used for pre-press color proofing are laser thermal printers, disclosed in U.S. Pat. No. 5,268,708, and inkjet color printers.
Each of these imaging technologies has inherent advantages and disadvantages. Laser thermal printing provides high-quality images that are often used as final proofs for emulating the output of a four-color offset printing system. Laser thermal printing uses laser energy to transfer colorant from a dye donor material to a receiver media. Because the laser printhead can focus a laser beam on an area of donor that is only a few microns in diameter, laser thermal printing is ideally suited for halftone dot reproduction on a color proof, emulating an offset printer's halftone dots by "pixelization," printing onto a receiver medium a grouping of tiny, adjacent microdots that taken together give the appearance of a halftone dot. Because the exposure energy used for printing these microdots can be varied over a range of values, laser thermal imaging allows a printer to emulate an offset printer's ink density.
A limitation of laser thermal printing is that media costs are high due to the use of separate donors in addition to the receiver media. Dye donor material is typically provided in sheet or roll form with the colorant embedded on a film base and several different color sheets are used to print one image. Another problem with laser thermal printing is use of specialty colors, which are used for corporate identity logos and packaging. Specialty colors are separately formulated inks. Rather than the "subtractive" process, which uses Cyan, Magenta, Yellow, and black, or CMYK inks, specialty colors emulate colors in halftone color offset printing. Because of the number of specialty inks required in commercial printing, it would be impracticable to make rolls of dye donor material for all the specialty colors in use.
Inkjet printers are also used for color proofing. Inkjet printing operates by applying ink in tiny discrete droplets to a receiver. Inkjet devices may operate using A continuous flow of ink where droplets are continuously produced during printing and unneeded droplets are deflected into a waste collector, or "drop-on-demand" printing wherein droplets are emitted by the printhead only when needed. Inkjet imaging technology can be used for generating color proofs by emulating halftone dots, or by printing continuous tone color areas.
Inkjet imaging, however, does not offer the advantages of variable density afforded by varying exposure energy with laser thermal printing. However, inkjet has other advantages, including generally lower media costs. A significant advantage of inkjet technology is that specialty color inks can be formulated at lower expense than is possible for laser thermal technology. A comparison of laser thermal and inkjet printing shows that the strengths of one technology often complement the weaknesses of the other.
Color proofing saves customers time and money when preparing high-quality printed materials. The more closely a color proof emulates the end-result of a printing press, the more likely a print job will run smoothly, minimize waste, and provide customers with a pleasing product. The final proof is typically treated as a contractual instrument, to be carefully examined and approved by the customer before the costly process of printing system setup and operation is initiated.
For high-quality print jobs, color proofing typically proceeds in stages. Early in the pre-press process, a "draft-quality" color proof may be sufficient for establishing final layout arrangement and overall appearance. As pre-press work progresses, successively better, intermediate-quality proofs are often desirable for showing the effectiveness of a color image and for refining its appearance. Then, as a job nears completion and is ready for final sign-off by the customer, a high-quality proof is needed, to show, as closely as possible, how the job will print. To match the workflow requirements of this process, a pre-press operator may prepare an early "draft-quality" proof inexpensively, using a low-cost inkjet printer. Then, for the final proof, the pre-press operator may prepare a final quality proof on a high-quality laser thermal printer. It would be advantages if a single printer could both provide draft and intermediate quality color proofs as well as a final color proof.
It can be appreciated by those familiar with digital imaging that, for both laser thermal and inkjet printers, the mechanical subsystem needed for handling paper or other receiver media must be able to feed the media correctly from a source roll or sheet feeder to a writing mechanism, and to support the media securely during printing for accurate resolution. The method predominantly employed for large-format printers is to mount the receiver on an imaging drum and use vacuum to attach the media to the imaging drum for printing. Thus, for acquiring and supporting the receiver for printing, the media handling subsystem for a laser thermal printer must perform many of the same tasks as the media handling subsystem for an inkjet printer.
It will also be appreciated by those familiar with digital imaging that the mechanical subsystem needed for printing a proof using an inkjet printhead must also perform the same tasks as the mechanical subsystem for printing a proof using laser thermal technology. For both, a printhead is passed over the surface of a receiver and the image is applied, either directly to the receiver or to an intermediate. The imaging drum rotates as the printhead moves in a line along the imaging drum parallel to the drum axis, applying the image to the receiver in a helical swath. It would be more efficient to use the same precision printhead positioning mechanism to perform both laser thermal imaging and inkjet imaging.
It is known that the use of multiple printheads in a single printer can provide certain advantages. Using multiple printheads of the same type, using the same printing technology, has been a strategy employed to boost printer efficiency. U.S. Pat. No. 5,677,719 (Granzow) teaches use of multiple inkjet printheads, each printing on a specific area of a receiver to increase printer speed and facilitate ink drying. U.S. Pat. No. 5,184,900 (Eisner et al) discloses a high-volume, high-speed printer having multiple dot matrix printheads to allow concurrent printing of an address and a bar code on envelopes for mailing. U.S. Pat. No. 5,488,397 (Nguyen et al.) discloses an arrangement of multiple inkjet printheads to effectively provide a wider print swath for improved printer throughput.
In addition to improving efficiency, multiple printheads have also been employed to improve image quality. As an example, multiple identical inkjet printheads are employed for pixel interleaving, effectively increasing the resolution available from a printer, as disclosed in U.S. Pat. No. 5,889,534 (Johnson et al.) and in U.S. Pat. No. 5,428,375 (Simon et al.). U.S. Pat. No. 5,764,254 (Nicoloff, Jr. et al.) discloses a printer having multiple inkjet printheads with different resolutions, wherein a black printhead is at a higher resolution than a color printheads, to provide black text characters at a higher resolution than is available for color inks.
There are other image quality benefits when a printer uses two or more printheads of different types, wherein each printhead has specific advantages for its intended use. For example, U.S. Pat. No. 4,595,303 (Kuzuya et al.) discloses a monochrome printer with a first type-printing printhead for producing crisp, clear text characters and a second dot matrix printhead for printing raster images or providing alternate font characters on the same output sheet. U.S. Pat. No. 5,167,456 (Murakoshi et al.) discloses a color thermal printer having a first text character printer using a black ink film and a second thermal wax transfer printer for printing Cyan, Magenta, and Yellow colors onto the same output sheet. U.S. Pat. No. 5,081,596 (Vincent et al.) discloses a text and color image printing system in which a first inkjet printhead applies color and a second laser printer prints text onto the same output sheet. U.S. Pat. No. 5,785,435 (Koo) discloses a text and color image printing system in which a first dye sublimation printhead prints a color image and a second inkjet printer or laser printer prints text on the same output sheet. U.S. Pat. No. 5,611,629 (Paranjpe) discloses a printer that employs a first dye-diffusion thermal printhead for printing Cyan, Magenta, and Yellow colors and a second thermal ink transfer printhead for printing black on the same output sheet.
While printers having multiple printheads are known, no printers combine the advantages provided by a laser thermal printhead and an inkjet printhead. The printers disclosed in the patents noted above use multiple printheads to print to the same receiver. None of these printers provide the option to print the same color image using either one printhead or the other. There is no option to print on a first receiver using the first printhead, and on a second receiver using a second printhead, while also allowing the option to print on a third receiver using both first and second printheads. None of the printers disclosed above employs the same printhead translation subsystem for both first and second printheads. The patents listed above require separate printhead stations and, in some cases, even separate receiver handling apparatus for applying the image to the receiver.
It would be advantageous to provide a printer that combines the advantages of both a laser thermal printhead and a inkjet printhead housed within a single apparatus.